How to Accurately Count Child Records in a LINQ Left Join and Group By?

LINQ – Tips for left joins, grouping and precise counting

Converting complex SQL queries to LINQ to SQL can sometimes be tricky. A typical example is to perform a left join, group by column and count the number of records that meet a certain condition.

Consider the following SQL query:

<code class="language-sql">SELECT p.ParentId, COUNT(c.ChildId)
FROM ParentTable p
  LEFT OUTER JOIN ChildTable c ON p.ParentId = c.ChildParentId
GROUP BY p.ParentId</code>

This query retrieves the ParentId and matching ChildId count from two tables, ParentTable and ChildTable.

To convert this to LINQ to SQL, first do a left join:

<code class="language-csharp">from p in context.ParentTable
join c in context.ChildTable on p.ParentId equals c.ChildParentId into j1</code>

However, the COUNT(c.ChildId) part needs to be handled carefully. Without special considerations, LINQ-generated SQL will always return COUNT(*), which may not provide the expected results.

To accurately count only matching ChildId values, add the following after the into clause:

<code class="language-csharp">from j2 in j1.DefaultIfEmpty()</code>

This ensures that if no matching child record is found, a null value is assigned to j2.

Finally, perform grouping and counting:

<code class="language-csharp">group j2 by p.ParentId into grouped
select new { ParentId = grouped.Key, Count = grouped.Count(t => t.ChildId != null) }</code>

This code groups by ParentId and only counts records whose ChildId is not empty. The result is an object with ParentId and Count properties, matching the format of the original SQL query. This ensures that we only count the cases where sub-records exist, avoiding null values ​​caused by left joins that affect the accuracy of the counting results.

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